He played college football for the Kansas Jayhawks and was selected by the Dallas Cowboys in the fourth round of the 2018 NFL draft.

Armstrong attended North Shore Senior High School, Galena Park ISD in Houston, Texas, where he played defensive end.

A 3-star defensive end recruit, Armstrong accepted a football scholarship from the University of Kansas, committing to Kansas over offers from California, Indiana, Iowa State, Michigan State, Purdue, and Texas Tech, among others.

Armstrong was selected by the Dallas Cowboys in the fourth round (116th overall) of the 2018 NFL Draft.

Entering the league as a 21-year-old rookie, he finished the season with 13 tackles, 8 solo, and a half of a sack, while playing in fifteen of the Cowboys sixteen games (one start).

In Week 10 against the Atlanta Falcons, he started in place of an injured Randy Gregory, making 3 tackles, one sack, 3 quarterback hurries and one block punt.

In the thirteenth game against the Washington Redskins, he had six tackles and returned a fumble recovery 37 yards.

In 2023, he appeared in all 17 games (one start), tallying 28 tackles (7 for loss), 7.8 sacks (second on the team) and 12 quarterback pressures.

On March 14, 2024, Armstrong signed a three-year contract with a maximum value of $45 million with the Washington Commanders.